Power steering apparatuses are conventionally utilized which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the rotational speed of the electric motor. Since the steering assist force is not required when the steering wheel is not turned, a so-called "stop-and-go control" is conducted, wherein the electric motor is off when the steering assumes a straight travel steering state virtually at a steering angle midpoint and, in response to detection of a steering angle change greater than a predetermined level, the electric motor is actuated.
The detection of the steering angle midpoint is, for example, based on an output of a steering angle sensor provided in association of the steering mechanism. The steering sensor is adapted to detect a steering angle change with respect to an initial steering angle, i.e., a relative steering angle. The initial steering angle corresponds to a steering angle observed when an ignition key switch is actuated. Accordingly, the initial steering angle does not always correspond to the steering angle midpoint, but often corresponds to a steering angle at which the steering wheel is offset from the steering angle midpoint. Therefore, steering angle data are sequentially sampled for determination of frequency of occurrence of steering angle data, and the most frequent steering angle data is defined as steering angle data corresponding to the steering angle midpoint, for example.
On the other hand, a greater steering assist force is required at low speed traveling than at high speed traveling. There has been a proposal that the electric motor is actuated in response to a slight steering angle change (e.g., 3 degrees) when the vehicle speed is lower than 10 km/h, and actuated in response to a greater steering angle change (e.g., 9 degrees) when the vehicle speed is not lower than 10 km/h. Thus, a catch-up (caught feeling) can be suppressed particularly at the low speed traveling.
However, no steering assist force is required when the steering angle is close to the steering angle midpoint. Accordingly, if the electric motor is actuated in response to a slight steering angle change when the vehicle speed is lower than 10 km/h, the steering assist is provided in excess, resulting in an energy loss.
It may be considered to employ a steering angle based electric motor actuation control in combination with a vehicle speed based electric motor actuation control.
However, the steering angle midpoint is not determined immediately after the startup of the vehicle engine, but the determination thereof becomes possible after the vehicle travels a certain distance. Hence, the absolute steering angle cannot be detected during a certain period immediately after the startup of the engine, so that the electric motor actuation control cannot be performed on the basis of the steering angle.
For this reason, the electric motor is actuated in response to a slight steering angle change at the low speed traveling irrespective of the steering angle. Therefore, improvement in the steering feeling comes first with the energy saving sacrificed to some extent.